How to handle this problem with seller? Item was never delivered.

[volcs0]

I sold a DVD box set from Amazon Marketplace. The cost of the item was $150, and it was shipped media mail. The tracking number says delivered, but the buyer says he never received the item. The only conclusion we can draw is that the mail carrier delivered the item to the wrong address, and now it is gone. I've checked my database, and the address I used was the correct one. My record at stamps.com confirms that the correct address was on the package.

The seller wants a full refund, and while I see his point, it is a lot of money.

Without insurance, do I have any recourse against the USPS? What if the buyer is a scam artist? If the item was truly delivered to a wrong address, then as the buyer, I too would demand a refund.

Any thoughts?